The present invention relates to improvements in extrusion machines and to a method for improving the performance of such machines in extruding meltable compositions, such as polymers, metals, and other extrudable materials more effectively and efficiently.
Conventional extrusion machines include those of the type illustrated by my U.S. Pat. No. 4,344,710. Such machines generally have a barrel and screw ratio of length to diameter of at least 16:1 and generally from 24 to 36:1, to accommodate a feeding zone, compressing zone, heating zone(s), mixing zone, and cooling zones(s). The solid extrusion composition is fed into the feed zone (throat) in powder, granular, or pellet form, where it enters the screw channel and rotates with the screw during axial movement to the compression zone. Such rotation of the solids bed with the screw inside the smooth barrel produces undesirable frictional heat generation which softens the solids and limits or prevents the axial movement of solid material. To prevent this frictional heat build-up, the screw and barrel are generally water cooled in the throat and feed zones.
Extruders with grooved barrels and throats are used by the plastics industry because they forward the solids axially with the screw and resist or prevent the solids from rotating with the screw inside the barrel. These grooved barrel extruders use energy more efficiently than smooth bore extruders in melting and pumping the composition. However, there appears to be a practical size limitation for the screw diameter, about 31/2 inches. Barrel and screw cooling is also critical on these grooved barrel extruders because elevated temperatures prematurely soften the extrudate in the barrel grooves allowing the softened solids to turn in the barrel with the screw rather than advancing in the axial direction. The typical axial groove in a conventional grooved barrel is constructed with parallel sides spaced apart by about 2 diameters of a plastic resin pellet and have an average groove depth approximately equal to the groove width. The depth of these grooves is about 11/2 times their width in the feed throat zone and taper to no depth at the end of 6 to 9 barrel diameters of length. These typical grooves are difficult to clean when they plug or bridge because of interrupted cooling or excessive back pressure.